UNIVERSITY   OF   CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL    EXPERIMENT   STATION 

BERKELEY,    CALIFORNIA 


CROP  SEQUENCES  AT  DAVIS 


JOHN  W.  GILMORE 


BULLETIN  393 

October,  1925 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1925 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://www.archive.org/details/cropsequencesatd393gilm 


CROP  SEQUENCES  AT  DAVIS 


JOHN  W.  GILMORE* 


The  purpose  of  this  bulletin  is  to  set  forth  the  trends  of  the  yields 
of  some  cereal  crops  under  several  rotation  practices  in  relation  to 
certain  soil  and  climatic  conditions  and  the  use  of  green  manures. 
The  data  cover  the  ten-year  period  from  1914  to  1923  inclusive ;  and 
for  convenience  of  interpretation  the  results  of  the  first  and  second 


(jsiWEflsrry  of  i:auh»ma 

fORNSA  I        ■       tOUEGE   OF     Al.B!ClSLTt;*£ 

-Twt      mm     ■  '■"».;  *\,.w 

»BCf^      J.        '  j&B-f 

Ik ?  —  ^LBI,  '     -^k     w i4 

Fig.  1. — Continuous  production  of  the  same  crop  depletes  the  yield. 

1.  Wheat,  fallow,  barley,  peas.. ..average  yield  42.3  bushels  per  acre 

2.  Wheat,  alternate   fallow average  yield  40.0  bushels  per  acre 

3.  Wheat    alternate    vetch average  yield  38.7  bushels  per  acre 

4.  Wheat    continuously average  yield  22.6  bushels  per  acre 

five-year  periods,  where  possible,  are  shown  in  comparison.  Obviously 
a  ten-year  period  for  data  of  this  kind  is  not  sufficient  for  definite 
conclusions  to  be  drawn  in  all  instances.  As  time  goes  on  the  data 
will  become  more  comprehensive  and  have  increased  significance.  It 
is  believed,  however,  that  the  data  herewith  presented  show  clearly 
the  general  trends  of  certain  practices  under  the  conditions  existing 
at  Davis. 


Division  of  agronomy. 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE    1 

Arrangement   of  Plots,   Their   Treatment  and   Yields 
Bu.  Per  Acre 


Plot 
No. 


Wheat  continuously.. 

Wheat,  cultivated- 
fallow 

Wheat,  cloddy — 
fallow 

Check 

Wheat,  weedy — 
fallow 

Barley  continuously. 

Check 

Rye  continuously 

Oats  continuously 

Check 

Wheat— manure  con- 
tinuously  

Milo  continuously — 

Check 

Saccari  ne— sorgh  um 
continuously1 

Wheat,  wheat,  fallow 

Check 

Wheat,  fallow,  wheat 

Fallow,  wheat,  wheat 

Check 

Wheat,  wheat,  wheat, 
fallow 

Wheat,  barley,  fallow 

Check 

Oats,  wheat,  barley, 
fallow 

Barley,  milo,  wheat, 
fallow 

Check 

Milo,  wheat,   fallow, 
barley 

Wheat,    fallow,    bar- 
ley, milo 

Check 

Fallow,  barley,  milo, 
wheat 

Barley,  peas,  wheat, 
fallow 

Check.. 

Peas,  wheat,  fallow, 
barley 

Wheat,    fallow,    bar- 
ley, peas 

Check 

Fallow,  barley,  peas, 
wheat 

Wheat,  peas 


13-14 


14.5 


19.3 
11.3 


13.6 
50.4 


61.2 
14.1 


79. 4€ 
16.3 

7. 
9.3 
11.3 
8.3 


9.1 

11.6 
11.3 
10.6 

59.3 

63.6 

12.3 

64.64 

17.8 
17.6 


58 
21.3 


3.34' 
22.3 


26. 


30 


14-15 


23.67 


24.83 


40.80 

26. 

21.5 

51.56 

22.33 

23 

86.62 

20.83 

12.6 

17.33 

20.33 


18.17 
14.83 

13. 

36.4 

14.83 

18. 

78.21 
8.50 

13.50 


11.17 
37  40 


5.36 

7. 


27.33 


7.67 


15-16 


38.33 

51.67 

50.67 
38.33 

43.00 

60.60 

36 

12.16 

80 

39.33 

42.33 
61.67 
38.67 

7.73 


41. 
60.67 
48.33 
44 

54.67 


52.67 
51.67 


tO  4 
36.67 


20  20 


58.33 
32.33 


70.40 
25.67 


3.63 
44  67 


16-17 


41.50 


46.00 


74.2 
54 

38.6 
122.5 


45.33 
65.6 
48.5 

9.6 
57.16 
47.16 
51.33 


42 


53.66 
40.33 


43.66 
72.80 


35 


45.33 
43.6 


10.51 
32.16 


65 

7.25 


17-18 


25.8 
46.8 

44.3 

42 

42.3 
67.7 
38.8 
33.5 
84.3 
33.3 

39.3 
7.11' 
37.5 

5.62 
39. 
24.1 


49.3 
37. 

42.3 
71.2 
35  1 

79.6 

62.5 
38.8 


7 
32.6 


70.8 
25.3 


13.7  = 


49.3 
25.5 


52.3 


18-19 


28.7 


27.5 


27.4 

22 

14 

27.3 

32.2 

21.3 
26.6 
27.8 

3.12 


17.4 
17.4 
19.6 
12.5 

13.7 

13.7 


2.75 
12.1 


24.5 


13  5 


44  2 
3.75 


19-20 

3.7 

24.1 

20.3 
4.8 

33.9 
16.4 

3  3 

6.6- 

4.8 

2.4 

2.2 
2.3 
3. 

6. 

38.4 

1.8 

3.2 


2.2 

4. 

25 

2 

11.6 

3. 
2.3 


43.6 
4  6 


3  5 


31.1 

3.2 


41. 
3.8 


1.25 
30  5 


20-21 
20.6 


20  3 


25.2 
16.3 
20.8 
18.8 
18.6 

22.3 
43.1 
21.5 

7.6 
17.3 
14.3 


24  6 
21 


25.2 
20.7 


17.6 

49.6 

46.7 
11.6 

18.6 

13.9 

49.6 

.56 
12. 

17.6 


45 


21-22 


38. 

13.3 

63.3 

66.3 

26.7 

7.7 

57  7 

41. 
28  2 
30.7 
69.7 
30.7 

41  7 
46.3 
41. 

4.15 


29  8 
58.3 
45.7 
36.7 

69.7 

36.8 

85.6 

71. 

35.8 


52.2 
21.3 


65.6 

28.2 

4  22 

61.8 
29.3 


54  3 


22-23 


22.1 
9.5 
15. 
61.9 
10. P 

3? 

3b. 9 
24.6 

3.92 
71.3 
14.5 
23.2 


17 

24  8 
58.3 
13.5 

47.2 

39.3 
22.8 

68.3 


11.6 
74.4 
5.311 


Tons  green  matter  per  acre. 


Bull.  393 


CROP    SEQUENCES   AT    DAVIS 


TABLE  1—  (Continued) 


Plot 
No. 


50 


Check 

Peas,  wheat 

Wheat,  vetch 

Check 

Vetch,  wheat 

Wheat,    milo,    rye — 

rape 

Check 

Milo,  rye — rape, 

wheat 

Rye — rape,  wheat, 

milo 

Check 

Wheat,  milo,  rye — 

vetch 

Milo,  rye — vetch, 

wheat 

Check 

Rye — vetch,    wheat, 

milo 

Wheat-clover  con- 
tinuously  

Check 


13-14 


35 

3.34 
24 
22 

3.31 

24 
25.3 

56.78 

20.5 

33.3 

29.46 
24.6 


28.5 
24.6 


14-15 


15. 
18.60 
6.1i 
12.16 
15.92 

64.12 
10.50 

4.50! 

19.83 
13.58 

47.28 

11.33 
20.66 

23.66 

17.33 
21.66 


15- 


21  67 

8.47 

54.33 

29.35 

6.05 


28.33 
42 


22.19 
35 


42.67 
32. 


35.66 


5-17 


30.60 
39.33 
10. 251 
27.33 
36. 

35.66 

28 

90.3 


34 
36.5 


97.3 
39 


24.35 
28. 


17-18 


25.5 
11.91 

51.6 
25  5 
10' 

0 

23.8 

13.81 

40.1 
31.6 


35.1 
31.6 


38 


26.3 
31.1 


18-19 


16.9 
21.2 
1.75 
14.5 
21.2 

8.1 
16.1 

25.8 

60  6 
18.8 

6.37 

35 
21.4 

56.6 

15.8 
13.8 


19-20 


3.4 
1.5" 

30.9 
3. 
1.25 

28 
2  5 

6.3 

2.25 
4. 

34.6 

3.5 


2.12 


2.1 
3. 


20-21 


14.2 
20.5 
25 
12.8 
22.8 

40.9 
14.2 

3.69 

20.1 
16.4 

43.2 

2.75 
14.7 

18.1 

17.8 
12.7 


21-22 

28. 

5.12 
47.7 
25.7 

5  25 

4.37 
26.2 

55. 

78  8 
21.3 

7.34 

60.8 
29 

82.3 

36.7 
27.3 


22-23 


5.56 
15  5 


67 


42.9 
10.5 


15.7 
16.7 


1  Tons  green  matter  per  acre. 


2  Pasture. 


The  importance  of  this  field  of  investigation  is  emphasized  by  the 
fact  that  the  major  portion  of  the  land  area  in  farms  in  California  is 
not  irrigated  and  receives  a  rainfall  of  less  than  20  inches,  which  falls 
mainly  during  the  winter  months.  Under  such  conditions  the  eco- 
nomic diversification  of  crops  is  restricted,  and  the  maintainance  of 
the  crop  producing  power  of  the  soil  and  the  conservation  of  the  mois- 
ture supply  present  additional  difficulties. 

Plan  and  Procedure. — A  series  of  plots  was  laid  out  in  1913  on 
land  that  had  been  fallowed  the  previous  year  but  which  had  grown 
grain  continuously  for  thirty  years  or  more.  It  comprises  52  plots 
each  121  feet  long,  and  18  feet  wide,  %o  of  an  acre.  There  is  an 
open  cultivated  space  of  three  feet  between  the  plots.  The  arrange- 
ment is  such  that  a  check  plot  occurs  adjacent  to  each  treatment 
plot,  each  third  plot  being  a  check  and  these  checks,  18  in  all,  are 
planted  with  wheat  continuously. 

The  plan  comprises  continuous  cropping  with  wheat,  oats,  barley, 
rye  and  milo ;  continuous  wheat  with  a  legume ;  continuous  wheat 
with  manure ;  wheat  and  alternate  green  manure ;  rotation  including 
fallow,  a  four  course  rotation  with  and  without  green  manure  and 
several  minor  trials.     Table  1  shows  the  arrangement  of  treatments 


b  UNIVERSITY    OP    CALIFORNIA EXPERIMENT    STATION 

and  checks.  The  yields  of  grain  are  expressed  in  bnshels  per  acre, 
and  the  yield  of  legumes,  forage,  etc.,  in  tons  of  green  matter  per  acre. 
The  sequence  of  the  plots  in  the  table  is  that  of  their  arrangement  in 
the  field 

This  table  covers  a  period  of  ten  years  from  1914  to  1923.  It  is 
presented  for  general  reference  and  no  comment  upon  it  is  necessary 
here  except  to  point  out  that  the  yields  from  year  to  year  have 
varied  widely,  and  that  very  low  yields  occurred  on  most  plots  in 
1920.  This  was  a  very  dry  season  during  Avhich  the  rainfall  (8.94 
inches)  was  only  a  little  more  than  half  the  normal. 


m 


Fig.  2. — Kough  and  smooth  fallow.  The  yield  of  these  two  plots  over  a 
ten-year  period  has  been  practically  the  same,  though  the  rough  fallow  has 
dropped  a  little  more  rapidly  during  the  second  half  of  the  period.  See 
table  9,  page  18. 


The  grain  crops,  including  the  vetch  and  peas,  were  planted 
in  the  fall,  usually  in  December,  though  with  occasional  variation 
because  of  rainfall  conditions.  The  plots  summer  fallowed  were  plowed 
in  December  and  again  in  March  and  given  two  or  three  cultivations 
during  the  early  summer  (except  plot  5,  which  was  left  rough  or  cloddy 
each  year.  See  fig.  2).  The  alley-ways  between  the  plots  were  cultivated 
during  the  summer.  Sometimes  the  plots  have  shown  an  increased 
growth  along  these  open  ways,  but  the  yield  has  not  been  deducted 
for  this  because  all  the  plots  showed  practically  the  same  effect  and 
the  results  are  comparable. 


Bull.  393]  CROP    SEQUENCES   AT   DAVIS  7 

The  mile-  and  saccharine  sorghum  crops  were  planted  in  the 
spring  usually  about  April  15th,  thinned  to  8  inches  in  the  row 
and  cultivated  three  to  four  times  during  April  and  May.  The  rows 
were  spaced  three  feet  which  gave  six  rows  to  the  plot. 

CLIMATE  AND  LOCATION 

Davis  is  located  near  the  geographical  center  of  the  state,  13  miles 
from  Sacramento,  and  is  properly  a  part  of  the  Sacramento  Valley. 
Its  climate,  however,  with  special  reference  to  rainfall,  humidity  and 
temperature,  is  somewhat  modified  by  the  conditions  of  these  factors 
prevailing  over  the  Bay  region  of  San  Francisco.  This  is  especially 
noticeable  in  regard  to  temperatures,  for  northern  points  in  the 
Sacramento  Valley  have  a  higher  mean  temperature  during  the 
growing  months  than  does  Davis,  and  a  lower  mean  temperature 
during  the  winter  months.  Thus,  for  example,  the  mean  temperature 
for  Davis  and  Chico  are  for  the  months  of  April  to  September  in- 
clusive 70.6  and  73.5  degrees,  and  for  the  months  October  to  March 
inclusive,  54  and  52.8  degrees  respectively. 

Occasional  northern  winds  occur  at  all  points  in  the  Sacramento 
Valley  from  April  to  September,  and  these  are  sometimes  a  serious 
factor  in  crop  production.  They  are  both  dry  and  warm  and  hence 
during  the  early  summer  months  extract  a  good  deal  of  moisture  from 
the  soil,  and  if  the  grain  is  in  bloom  or  filling  they  effect  a  deleterious 
influence  on  pollination  and  consequent  yields.  The  relative  dryness 
of  these  winds  is  shown  by  the  evaporation  from  a  free  water  surface, 
and  a  single  instance  may  suffice  as  an  illustration. 

June   14-16,    1917,   average   temperature,    105;    evaporation,    .48    inches;    pre- 
vailing wind  S.W. 

June   18-20,   1917,  average  temperature,   107;    evaporation,    1.20   inches;    pre- 
vailing wind  N. 

Rainfall. — Both  temperature  and  rainfall  vary  widely  at  Davis, 
Temperature,  however  does  not  exert  so  great  an  influence  on 
cereal  yields  as  does  rainfall,  because  during  the  season  of 
crop  growth  the  temperature  is  at  no  time  above  or  below  that 
which  is  favorable  for  cereals.  The  rainfall,  however,  is  a  very  im- 
portant factor  in  crop  growth,  and  its  effectiveness  is  measured  not 
only  by  the  annual  amount,  but  also  by  its  distribution  over  the 
season.  Since  practically  all  of  the  rainfall  of  the  valley  areas  of 
California  falls  during  the  winter  months,  October  to  March,  the 
farmer  must  meet  a  special  set  of  problems,  in  moisture  conservation 
and  in  the  maintenance  of  the  optimum  physical  condition  of  the 


8 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


soil.  Because  of  this  peculiarity  of  the  rainfall  it  semes  best  to  pre- 
sent the  rainfall  data  according  to  season  rather  than  by  calendar 
years.     (See  table  2.) 

TABLE    2 

Eainfall  at  Davis,  1913-23 


July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

Seasonal 

12-13 

3.43 
9.17 
4.62 
11.01 
1.28 
.95 

.15 
4.35 
5.01 
1.93 
5.90 
3.59 

1.08 
.85 

1  69 

1.14 
.61 

3.10 

.77 

1.00 

91 

.11 

.51 
.82 

.36 

.60 

2.20 

.10 

.07 

.66 

13-14 

4.63 
.16 
.53 
.35 
.12 

7.44 

4.75 

6.03 

4.81 

.59 

28.70" 
20.05 
20.88 
14.11 
9.66 

14-15 

.71 
.03 

.46 

15-16 

►  93 .  40  Total 

16-17 

.02 

1.7 

.49 

18.68  Av. 

17-13 

18-19 

4.07 
.55 
.03 

.28 

2.19 
.31 
4.02 
1.62 
3  2) 
.53 

1.69 
2.61 
4.39 
4.39 
7.37 
.88 

2.49 
.37 
5.11 
2.29 
2  62 

7.12 
.76 
.32 

5.85 
.70 

1.54 
3.47 
1  24 
1.47 

.02 
.83 
.30 
.40 

2.22 

19. 40^ 
8.94 
17.17 
16.63 
17.86 

19-20 

.04 

20-21 

.04 

1.46 
.21 

1.56 
.40 

.26 
.40 
.10 

\  80  Total 

21-22 

16  Av. 

22-23 

23-24 

.35 

Normal  rainfall  for  Davis  is  17.23  inches. 

During  the  first  period  of  these  five  years  there  fell  93.4  inches,  average  18.68, 

1.45  above  normal. 
During  the  second  period  of  these  five  years  there  fell  80  inches,  average  16, 

1.23  below  normal. 

From  this  record  it  may  be  seen  that  the  rainfall  has  varied  during 
the  ten  years  mentioned  from  more  than  28  inches  for  the  season  1913- 
14  to  less  than  9  inches  for  the  season  1919-20.  This  variation  in 
seasonal  rainfall  has  had  a  marked  influence  on  the  crops  produced  on 
all  the  plots  of  this  experiment,  and  accounts  to  some  extent  for  the 
wide  variation  in  yields  shown  in  table  1.  During  seasons  of  very 
heavy  rainfall  the  winter  grain  and  legume  crops  are  injuriously 
affected  by  an  excess  of  water  in  and  on  the  soil,  and  during  seasons 
of  deficient  rainfall,  spring  growth  of  the  grain  crops  is  retarded  and 
the  summer  growing  crops  suffer  for  a  lack  of  moisture,  especially  on 
the  continuously  cropped  plots. 

Seasonal  and  Spring  Rainfall. — Annual  winter  growing  crops  are 
also  affected  by  the  amount  of  precipitation  during  certain  months. 
Thus,  yields  in  1914,  after  an  abundant  rainfall,  were  not,  for  most 
of  the  winter  grown  crops,  so  heavy  as  the  yields  of  1918  after  a  defi- 
cient rainfall,  and  this  despite  the  fact  that  the  1914  crop  had  the 
advantage  of  early  planting  (Dec.  1)  as  compared  with  late  planting 
(Jan.  24)  for  the  crop  of  1918.  The  figures  of  table  3  tend  to  illus- 
trate this  point. 

These  years  were  chosen  because  they  represent  about  equal  devia- 
tions from  the  normal  rainfall   (17.23).     The  years  1914  and  1918 


Bull.  393" 


CROP    SEQUENCES   AT    DAVIS 


illustrate  the  same  tendency,  but  their  rainfall  is  further  from  the 
normal,  both  above  and  below.  By  referring  to  table  3  it  will  be 
noted  that  in  1916,  a  year  of  above  normal  rainfall,  more  than  17 
inches,  or  84  per  cent  of  the  total,  fell  before  February  1st.  The 
earlier  part  of  the  year  had  more  moisture  than  was  needed  because 
of  the  nature  of  the  crops  and  their  culture,  but  this  abundance  of 
moisture  was  largely  lost  because  it  was  in  excess  of  what  could  be 
absorbed  and  retained.  In  1917,  a  year  below  seasonal  rainfall, 
about  7  inches  or  a  little  less  than  half  of  the  total  precipitation  fell 
after  the  first  of  February,  while  in  1916,  a  year  above  normal  seasonal 
rainfall,  only  3.28  inches  fell  after  that  date.  These  data  with  ex- 
tensive observations  tend  to  emphasize  the  fact  that,  granted  a 
reasonable  amount  and  distribution  of  moisture  during  the  winter 
months,  it  is  the  rainfall  of  the  spring  months  that  makes  abundant 
grain  crops.  Moreover,  late  spring  rains  are  nearly  always  accom- 
panied and  followed  by  cool,  humid  weather.  This  not  only  retards 
evaporation  and  consequently,  loss  of  moisture  from  the  soil,  but  also 
prolongs  the  ripening  season  of  grain  crops,  and  both  of  these  influ- 
ences enhance  the  yield  of  these  crops.  Thus  in  1916  the  mean  tem- 
perature for  the  months  March,  April  and  May  was  59.6,  while  that 
for  the  same  months  in  1917  was  56.3,  the  normal  being  58  for  the 
same  months. 

It  is  not  supposed,  however,  that  these  yields  are  consequent  upon 
rainfall  alone.  There  are  other  factors  involved,  such  as  date  of 
planting,  shattering,  lodging  and  prevalence  of  disease,  the  influence 
of  which  it  is  impossible  to  evaluate;  but  it  is  strongly  indicated,  at 
this  stage  of  these  experiments,  that  on  these  soils  and  between  reason- 
able dates  of  planting,  the  rainfall  is  the  most  influential  seasonal 
factor  involved  in  yield  of  the  crops  under  study. 

TABLE  3 

The  Influence  of  Seasonal  Distribution  of  Eainfall  on  Yield  of  Crops 

Grown   Continuously 


Seasonal  rainfall 

Spring  rainfall  Feb. -April,  inc 

Wheat— average,  all  checks  (18  plots) 

Oats  continuously,  plot  11 

Rye  continuously,  plot  10 

Barley  continuously,  plot  8 

Milo  continuously,  plot  14 

Wheat  and  manure,  plot  13 

Wheat  and  legumes,  plot  53....... 

Date  of  planting  winter  crops 


1916 


1917 


20.88 

14.11 

3.28 

,7.02 

35.35 

39.58 

80.00 

122.50 

12  16 

38.60 

60.60 

74.20 

61.67 

65.60 

42.33 

45.33 

21.00 

24.35 

March  5 

Dec.  26 

10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


SOILS 


#//////////, 


"Hi 


*'/////&///, 


wmw< 


2 


The  soils  over  which  these  plots  are  distributed  are  of  two  types — 
fig.  3 — Yolo  loam  and  Yolo  silt  loam.  These  soils  are  described 
by  the  Bureau  of  Soils1  as  follows :  ' '  The  Yolo  loams 
as  mapped  in  this  area  comprise  the  Yolo  loam  and 
Yolo  silt  loam.  The  Yolo  loam  consists  of  a  light- 
brown  or  brown  loam  of  medium  to  heavy  texture. 
Gravel  is  usually  absent  and  the  soil  as  a  rule  is 
friable  and  easily  tilled.  Considerable  variation  in 
the  subsoil  is  noted,  but  below  a  depth  of  24  inches 
it  often  grades  into  a  silty  loam  or  sandy  loam  of  a 
buff  or  brownish-yellow  color.  Clay  loam  or  clay  at 
times  constitutes  the  deeper  subsoil  and  gravelly  beds 
may  occur  at  a  depth  of  4  to  6  feet  long  along  the  site 
of  old  stream  channels.  Altogether  the  type  shows 
decided  variations  in  the  texture  of  both  soil  and 
subsoil,  as  indicated  by  borings  taken  at  close  range. 

' '  The  Yolo  silt  loam  is  friable  in  texture.  Typically 
it  consists  of  36  inches  of  light-brown  or  dark  grayish- 
brown  silt  loam  of  heavy  texture.  The  soil  frequently 
grades  into  yellowish-brown  substrata  of  silty  clay, 
clay  loam  or  similar  material. 

"Extreme  ranges  from  layers  of  sandy  loam  or 
sand  to  clayey  beds  are  encountered  in  the  subsoil  of 
a  single  boring,  but  on  the  whole  the  group  is  friable 
when  cultivated  and  retentive  of  moisture.  On  the 
other  hand,  the  soils  are  sufficiently  open  and  porous 
to  drain  quickly." 

* '  These  soils  usually  lie  a  little  more  elevated  than 
surrounding  soil  areas  and  are  usually  well  drained. 
Alkali  seldom  occurs  in  them.  They  are  adapted  to 
a  variety  of  crops,  but  their  yields  of  grain  are  notice- 
ably high  during  seasons  of  ample  rainfall. ' ' 

Mechanical  Analysis. — The  following  table  gives 
the  results  of  mechanical  analyses  of  samples  of  the 
soil  and  subsoils  of  the  Yolo  loams : 

Fig.  3_ — The  52  plots  in  this  rotation  cover  two  types  of 
soil,  the  Yolo  loam  (L)  and  the  Yolo  silt  loam  (Sil).  Each  is 
underlain  by  a  light  and  heavy  phase  of  subsoil  marked  L  and 
PI  respectively  below  the  line. 


'////////, 


'■WMM 


»///////////, 


■■'//////////, 


*4\W///A 


<fNKW/: 


Bull.  393] 


CROP    SEQUENCES   AT    DAVIS 


11 


TABLE  4 
Mechanical  Analysis  of  Yolo  Loams 


Fine 
gravel 

Coarse 
sand 

Medium 
sand 

Fine 
sand 

Very  fine 
sand 

Silt 

Clay 

Yolo  loam — Soil 

0.1 

0.4 

0.9 

9.5 

27.3 

43.3 

18.2 

Subsoil 

0.0 

0.4 

1.4 

14.3 

16.5 

49.3 

18.1 

Yolo  gilt  loam — Soil 

0.0 

0.1 

0.1 

1.2 

5.0 

69.1 

24.4 

Subsoil 

0.0 

0.0 

0.0 

1.7 

9.1 

61.3 

27  9 

The  Yolo  silt  loam  predominates  over  the  particular  area  under 
consideration;  in  fact  it  occupies  the  entire  area  except  plots  3  to  6 
inclusive  and  small  areas  in  other  parts  of  the  strip.  Reference  to 
figure  1  will  indicate  the  distribution  of  these  types  over  the  area 
occupied  by  the  plots. 

Yields  of  Wheat. — It  would  seem  that  on  the  area  under  consid- 
eration the  Yolo  loam  is  somewhat  more  productive  than  the  Yolo 
silt  loam.  The  average  yield  of  two  check  plots,  3  and  6,  which  are 
Yolo  loam  with  a  light  subsoil,  is  a  little  higher  over  a  period  of  ten 
consecutive  years  than  is  the  average  on  six  check  plots  covering  the 
silt  loam  with  light  subsoil.  The  following  table  illustrates  this 
point : 

TABLE  5 

Average  Yield  of  Wheat  Grown  Continuously  Over  a  Period  of  Ten  Years 

on  Yolo  Loam  and  Yolo  Silt  Loam — Bushels  Per  Acre 


Plot  treatment 

Average 

yield 
10  crops 

First 

half 

period 

Second 

half 
period 

Soil  type 

Average 
yields 

25.26 
24.95 

29.66 
32.49 

20.87 
17.40 

25  10 

22.17 
23.70 
24.06 

28.79 
29.52 
30  78 

15.50 
17.90 
17  30 

23  46 

Yolo  silt  loam 

20  07 
20.55 
19  26 

27  23 
26.26 
23.50 

12.80 
14  80 
15.00 

Yolo  gilt  loam. 

33  check — wheat  continuously 

36  check — wheat  continuously 

Yolo  silt  loam 

Yolo  silt  loam 

19.96 

It  is  true  that  the  average  of  six  plots  covering  Yolo  silt  loam 
(in  two  blocks)  is  compared  with  that  of  two  plots  (3  and  6),  cover- 
ing Yolo  loam.  If  the  two  groups  of  silt  loam  plots  are  averaged  there 
would  seem  to  be  a  significant  difference  in  productivity  in  favor  of 
the  Yolo  loam. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Moisture  Relationships. — There  are  several  variations  in  the  sub- 
soils of  this  area,  each  type  being  underlaid  in  places  by  light  and 
heavy  subsoils.  (See  fig.  2.)  But  in  no  cases  are  these  soils  imper- 
vious to  moisture,  hence  the  subsoils  do  not  influence  yields  to  any 
appreciable  extent.  The  moisture  holding  capacity  of  these  soils  is 
good,  and  except  during  periods  of  heavy  rainfall  their  absorptive 
capacity  is  sufficient.  In  general  the  rain  permeates  the  soil  readily 
and  standing  water  does  not  occur  on  the  surface  except  when  the 
very  heavy  rains  occur.  The  season  of  1915-16  is  typical  of  the  rela- 
tionship of  soil  moisture,  rainfall  and  yield. 

TABLE    6 

Average  Percentage  of  Moisture  by  Feet  in  the  Soil  of  18  Check  Plots  on 
Six  Different  Dates  for  the  Season  1915-16 


Depth 


Sept.  1, 1915 


Nov.  1,  1915 


Apr.  1,  1916 


June  1,  1916 


Sept.  1,  1916 


Nov.  1,  1916 


1  foot 

2  feet 

3  feet 

4  feet 

5  feet. 

6  feet 


6.32±118 
11  92±.453 
18.05±.819 
21.51±.896 
22.24±705 
26.64±.731 


6.25±.410 
12.35±577 
17.04±.812 
21.22±.490 
24.98±  841 
25.10±.752 


19.25±217 
22.73±.300 
26.65±473 
30.25±  370 
29.90=b  319 
32.71±346 


10  66±.377 
14.38±.211 
18.31±.471 
23.13±625 
28.03±.587 
29.07±.400 


8.47±221 
12.60±.304 
18.73±701 
23.76±670 
24.32±.536 
27.43±591 


8.67±.175 
13.96±.331 
19.36±.730 
22.11±.702 
25.94±.603 
28.00±.688 


The  total  rainfall  for  the  season  was  20.88  inches  (normal  17.23) 
and  the  average  yield  of  wheat  on  the  18  check  plots  was  for  the  har- 
vest of  1916,  35.23  bushels  per  acre  (average  for  10  years  22.63). 

Volume  Weight. — The  volume  weight  of  these  soils  shows  an  ap- 
proximation to  the  normal  for  these  types  of  soil  under  cultivation. 
The  average  volume  weight  of  the  first  foot  from  the  18  check  plots 
determined  from  samples  taken  about  November  1,  1924,  was  1.5174± 
.0479.  In  cases  where  organic  matter  has  been  added,  such  as  plot 
13,  where  manure  has  been  applied  (V.W.  1.294)  ;  plots  38  and  40 
(V.W.  av.  1.392)  where  peas  have  been  plowed  under  and  plots  1,  41 
and  43  (V.W.  av.  1.447),  where  vetch  has  been  plowed  under,  the 
volume  weight  has  been  increased  and  there  is  a  corresponding 
increase  in  yield  of  wheat  over  that  of  the  check  plots. 

These  results  are  corroborated  by  observation.  In  plowing  the 
plots,  it  has  been  noticed  during  recent  years  that  in  general  the  check 
plots  plow  with  greater  difficulty  than  do  those  fallowed  or  those  on 
which  organic  matter  has  been  applied.  This  observation  does  not 
hold,  however,  for  the  sorghum  plots,  for  these  have  also  broken  up 
with  greater  difficulty  than  the  fallowed  or  the  manured  plots.  This 
is  probably  due  to  certain  physical  conditions  brought  about  in  the  soil 


BULL.  393]  CROP   SEQUENCES   AT   DAVIS  13 

by  the  influence  of  the  growth  and  decay  of  the  sorghum  roots  and 
stubble.  That  this  is  true  is  indicated  by  recent  investigations  by 
Breazle2  and  Hawkins,3  who  have  found  that  a  sorghum  crop 
leaves  the  soil  in  poor  physical  condition  as  compared  with  corn  and 
some  other  crops. 

CROP  SEQUENCE 

One  of  the  oldest  observations  in  agricultural  practice  is  the  influ- 
ence of  a  preceding  on  a  succeeding  crop.  It  has  long  been  known  that 
certain  legumes  have  a  beneficial  influence  on  crops  succeeding  them ; 
and  sometimes  on  crops  accompanying  them.  That  intertilled  crops 
are  beneficial  to  broadcasted  crops  following  them,  and  that  poor  lands 
remaining  in  sod  for  a  few  to  many  years  are  improved  in  physical 
condition  and  producing  power,  have  also  long  been  noted. 

These  observations  can  generally  be  explained  in  the  light  of 
present-day  knowledge  by  one  or  more  of  the  following  reasons: 
Legumes  are  beneficial  to  crops  following  them  because  of  the  available 
nitrogen  which  the  legumes  fix  in  the  soil  and  because  of  their  ame- 
liorating effects  relating  to  organic  matter.  Cultivated  crops  are 
beneficial  because  through  tillage  the  physical  condition  and  moisture 
relationships  of  the  soil  are  improved  for  the  succeeding  crops.  Where 
poor  lands  are  improved  by  remaining  in  sod  continuously  for  several 
years  the  benefit  is  due  to  an  increase  of  organic  matter  in  the  soil 
which  improves  its  physical  condition.  In  these  experiments  some 
evidence  has  been  obtained  of  the  influence  of  crops  in  sequence,  es- 
pecially in  respect  to  yield. 

Effect  of  MiJo  on  Succeeding  Crops. — During  recent  years  it  has 
been  very  definitely  shown4  that  in  some  instances  one  crop  leaves 
substances,  the  products  of  decay  of  roots  and  plant  parts,  which  are 
toxic  to  plants  or  crops  following.  One  of  the  most  recent  investi- 
gations2 on  this  subject  shows  that  decaying  sorghum  stubble  has  a 
poisonous  effect  on  wheat,  oats  or  barley  following.  After  presenting 
significant  experimental  data  the  author  of  this  investigation  states : 
"It  appears  that  a  toxic  compound  is  developed  in  sorghum  stubble 
during  the  process  of  decomposition''  and  "it  would  seem  that  the 
toxic  effect  might  be  more  pronounced  upon  heavy  soils  or  upon  soils 
with  heavy  subsoils  and  upon  soils  with  poor  drainage  than  upon 
sandy  soils  or  soils  with  good  drainage. ' ' 

Wheat  after  Milo,  on  the  plots  under  study,  has  shown  some  dimi- 
nution of  yields,  and  the  diminution  would  probably  have  been  even 
greater  had  the  soils  been  heavier  or  the  drainage  poorer. 


14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  average  yield  of  nine  crops  of  wheat  following  milo  was  30.27 
bushels,  whereas  wheat  continuously  on  the  18  checks,  produced  an 
average  yield  of  22.63  bushels  and  wheat  after  peas,  as  an  average  of 
18  crops,  produced  40.31  bushels  per  acre.  There  is  much  evidence  to 
indicate  that  a  cultivated  crop  preceding  a  wheat  crop  should  not 
have  a  tendency  to  reduce  the  wheat  yield  so  greatly  as  in  this  case. 


Fig.  4. — For  various  reasons  wheat  after  milo  does  not  do  so  well  as  wheat 
following  some  other  crops.  Plot  on  left  (28)  is  wheat  after  milo,  yield  (1919) 
20  bushels  per  acre.  Plot  on  the  right  (27)  wheat  after  wheat  (check)  18 
bushels  per  acre.     See  table  20. 

Although  the  beneficial  or  baneful  influence  of  one  crop  on  an- 
other has  long  been  recognized ;  yet  because  of  many  influences,  such 
as  climate,  soil,  topography  and  the  like,  no  fixed  code  of  facts  bearing 
upon  crop  sequence  has  been  drawn  up.  The  following  data  and  dis- 
cussion are  presented  with  the  belief  that  they  indicate  general  trends 
rather  than  definite  conclusions. 

Effect  of  Continuous  Cropping  on  Yield. — Perhaps  no  fact  in 
agricultural  practice  is  better  established  than  the  injurious  effects 
that  ultimately  result  from  the  practice  of  growing  the  same  crop  on 
the  soil  continuously.  This  is  a  general  fact  that  may  be  observed  in 
all  agricultural  regions  and  over  all  periods  of  time.  The  rapidity  of 
the  deterioration  depends  on  many  conditions  and  these  vary  under 
different  circumstances,  such  as  crop  grown,  inherent  quality  of  the 
soil,  and  climatic  factors.    The  reasons  for  the  deterioration  are  many, 


Bull.  393]  CROP   SEQUENCES   AT   DAVIS  15 

and  much  remains  to  be  learned  about  the  factors  involved.  It  must 
be  remembered  too,  that,  in  the  great  valleys  of  Central  California, 
seasonal  rainfall  has  a  great  influence  on  crop  yields,  hence  inter- 
pretation of  the  results  shown  is  complicated  and  it  is  impossible  to 
determine  the  exact  measure  of  emphasis  that  should  be  placed  on 
seasonal  rainfall  and  on  soil  conditions. 


Fig.  5. — Crop  of  1920.  Continuous  wheat  on  the  left,  wheat  plus  5  tons  of 
manure  per  acre  annually  on  the  right.  Yields  2.4  and  2.2  bushels  per  acre 
respectively.  This  amount  of  manure  seems  to  have  a  depressing  effect  on  growth 
and  yields  of  wheat  during  dry  years. 

In  1916  Madson6  reported  on  the  yield  of  wheat  produced  continu- 
ously on  the  same  land  for  a  period  of  four  years.  The  average  of 
eight  crops  thus  produced  (plots  duplicated)  was  11.38  bushels  per 
acre.  This  yield,  however,  might  have  been  greater  had  it  not  been  for 
a  subnormal  rainfall  during  three  years  of  the  period.  The  present 
experiments  are  a  continuation  and  an  enlargement  of  those  reported, 
and  cover  a  longer  period. 

The  influence  of  continuous  cropping  on  the  yield  of  wheat  can  not 
be  fully  determined  by  the  data  at  hand,  for  the  period  covered  by 
the  experiment  has  not  been  long  enough  and  there  are  other  factors, 
such  as  rainfall,  prevalence  of  wild  oats,  time  of  planting  and  the  like, 
the  exact  bearing  of  which  cannot  be  measured. 


16 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Enough  data  is  at  hand,  however,  to  indicate  the  trend  of  this 
practice.  Thus  table  7  shows  the  yields  of  the  18  check  plots  over 
the  ten-year  period,  making  in  all  180  trials. 


TABLE  7 

Summary  of  Yields  of  Check  Plots,  Wheat  Continuously,  for  10  Years, 
1914-23  Inclusive,  Bushels  Per  Acre 


Plot 

1914 

1915 

1916 

1917 

1918 

1919 

1920 

1921 

1922 

1923 

Aver- 
age 

1st 

half 

of 

period 

2nd 

half 

of 

period 

3 

6 

9 

12 

15 

18 

21 

24 

27 

30 

33 

36 

39 

42 

45 

48  .... 

51 

54 

19. 
11.3 
19 

14  1 
16.3 
11.3 
9.1 
10.6 
12.3 
17.6 
21.3 
26.6 
35 
22 

25.3 
20.5 
24.6 
24.6 

23.67 

24.83 

26. 

22  33 

20.83 

20.33 

14.83 

14.83 

8.50 
11.17 

7. 

7.67 
15. 

12.16 
10.56 
13.58 
20.66 
21.66 

38.33 

38.33 

36. 

39.33 

38.67 

41. 

44. 

53. 

51.67 

36.67 

32.33 

25.57 

21.67 

20.35 

28.33 

35 

32 

24 

41  5 

46. 

54. 

46.16 

48.5 

47  16 

42.66 

40.33 

43.66 

38. 

45  33 

32.16 

30.6 

27.33 

28. 

34 

39 

28 

25.83 

42 

38  83 

33.33 

37.5 

24.16 

37 

35.16 

38.83 

32.7 

25.33 

25.5 

25.5 

25.5 

23.83 

31.66 

31.66 

31.66 

28  7 

27.5 

22. 

32.2 

27.8 

17.4 

12.5 

13.7 

18. 

15. 

12.1 

13  5 

16.9 

14.5 

16.1 

18.8 

21  4 

13.8 

3.7 

4.8 

3  3 

2.4 

3. 

1.8 

2.2 

2. 

2.3 

4.6 

3.2 

3.8 

3.4 

3. 

2.5 

4. 

4. 

3. 

20.6 

20.3 

16.3 

18.6 

21.5 

14.3 

21. 

20.7 

17.6 

11.6 

13.9 

12. 

14.2 

12.8 

14.2 

16  4 

14.7 

12.7 

38. 

26.7 

28 .2 

30.7 

41. 

29.8 

36.7 

36.8 

35.8 

21.3 

28.2 

29.3 

28 

25.7 

26  2 

21.3 

29 

27.3 

13  38 

7  7 

9.5 

10.6 

24.6 

14  5 
17. 
13.5 
22.8 
11.6 
16.8 
16.5 
15.3 
17. 
25. 
15.5 
10  5 
16.7 

25.26 
24.95 
25.51 
24.98 
27  97 
22.17 
23.70 
24.06 
25.15 
20.07 

20  55 
19.26 
20.56 
18.93 
20. 

21  07 
22.75 
20.34 

29.66 

32  49 
34.76 
31.05 
32  36 
28.79 
29.52 
30  78 
30  99 
27.23 
26.26 
23.50 
25  55 
21  46 
23  20 
26.95 
29.58 
25.98 

20.87 

17.4 

15.86 

18.92 

23  58 

15.56 

17.88 

17.34 

19.3 

12.82 

14.84 

15.02 

15.56 

14.60 

16.80 

15.2 

15.92 

14.70 

Ave. 

18.92 

±.992 

16.42 
±.946 

35.35 
±1.43 

39.58 
±1.18 

31.43 

±921 

19. 

±965 

3.2 
±.426 

16.3 
±.559 

30. 
±804 

15  47 

±755 

22  63 

±469 

28.34 

16.79 

It  will  be  observed  that  there  are  wide  variations  in  yield  both  as 
between  years,  and  between  plots.  However,  the  general  trend  has 
been  downward  with  the  passing  of  time.  The  grand  average  yield 
for  all  the  check  plots  for  the  entire  period  is  22.63  bushels  per  acre. 
This  is  a  little  higher  than  the  average  for  the  state  for  the  ten-year 
period  covering  the  same  years,  which  was  17.14  bushels  per  acre. 
When  it  is  remembered  that  a  large  part  of  the  wheat  in  California 
is  grown  on  fallow  land,  which  should  enhance  the  crop,  it  will  be 
seen  that  the  effects  of  continuous  cropping  on  the  area  under  study 
are  not  yet  in  the  state  of  unprofitable  production.  It  requires,  one 
year  with  another  and  under  a  wide  range  of  prices  and  labor  condi- 
tions, about  14  bushels  of  wheat  per  acre  to  pay  the  cost  of  production. 

Comparing  the  first  half  of  the  period  with  the  last  half,  however, 
the  trend  of  production  does  not  appear  so  favorable.  Thus,  during 
the  first  five  years  of  the  period  the  average  yield  on  the  18  check 
plots,  representing  90  trials,  was  28.34  bushels  per  acre,  while  these 


Bull.  393] 


CROP    SEQUENCES   AT    DAVIS 


17 


same  plots  produced  during  the  last  5  years  of  the  period  only  16.79 
bushels  per  acre. 

Rainfall  in  Relation  to  Decreasing  Yield  Under  Continuous  Crop- 
ping.— These  figures  must  not  be  taken  as  expressing  the  exact  produc- 
ing power  of  these  plots,  for  a  rainfall  factor  is  involved.  Between  the 
two  halves  of  the  period  the  rainfall  varied  widely.  Thus  during  the 
first  half  of  the  ten-year  period  the  total  rainfall  for  each  season 
averaged  18.68  inches,  which  is  above  normal  by  1.45  inches,  while 
that  of  the  latter  half  of  the  period  amounted  to  only  16  inches,  which 
is  1.23  inches  below  normal.  Each  half  of  the  ten-year  period,  how- 
ever, had  a  year  of  very  low  rainfall  (1917-18,  9.66  in.)  and  (1919-20, 
8.94  inches).  The  rainfall  difference  is  even  more  striking  when  that 
falling  during  the  spring  months  of  February-May  inclusive  is  taken 
into  account.  During  these  months  for  the  first  five-year  period  the 
average  rainfall  was  8.21  inches,  while  for  the  same  months  during  the 
second  five-j^ear  period  the  average  rainfall  was  only  3.60  inches.    ■  ' 

The  yields  of  these  check  plots  have  also  been  modified  by  the  pres- 
ence of  wild  oats,  the  amount  of  which  does  not  show  in  the  figures  for 
yields.  Wild  oats  mature  ten  days  to  two  weeks  earlier  than  the  wheat. 
Consequently  the  yield  of  oats  does  not  appear  in  the  figures  given. 

Exactly  what  significance  should  be  placed  upon  the  influence  of 
this  subnormal  seasonal  and  spring  rainfall  upon  the  yield  of  wheat 
during  the  second  five-year  period  and  upon  the  presence  of  wild 
oats  is  difficult  to  determine.  But  the  data  at  hand  point  clearly  to 
the  declining  producing  power  of  the  soil  under  continuous  cropping. 

The  same  general  trends  are  apparent  under  continuous  cropping 
to  oats,  barley,  rye  and  milo.  The  following  table  illustrates  this  point. 
In  the  experiment  under  consideration  one  plot  was  continuously 
planted  with  each  of  these  four  grains,  and  the  yields  in  bushels  per 
acre  are  tabulated  as  follows : 

TABLE    8 
Influence  of  Continuous  Cropping  on  Barley,  Eye,  Oats  and  Milo 


Average 
10  crops 


Average  - 
lat-5  crops 


Average - 
2nd-5  crops 


Plot  8,  continuous  barley. 

Plot  10,  continuous  rye 

Plot  11,  continuous  oats... 
Plot  14,  continuous  milo. 


42.58 
24.09 
79  45 
52.37 


58.74 
30.75 
79.91 
73  34 


26.42 
17.42 
78.98 
35  60 


It  is  probable  that  the  rapid  decline  in  the  second  period  is  due 
largely  to  the  lower  seasonal  and  spring  rainfall  during  that  period 
and  to  the  fact  that  the  second  of  two  closely  following  dry  years  fell  in 


18 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


that  period.  For  some  reason  which  is  not  fully  apparent  oats  have 
not  shown  the  decline  in  yield  shown  by  the  other  crops.  The  yield 
of  oats  from  year  to  year,  however,  has  varied  over  a  wide  range, 
being  above  100  bushels  per  acre  two  years  and  below  30  one  year. 

Influence  of  the  Summer  Fallow. — Three  plots  in  this  experiment 
are  devoted  to  alternate  wheat  and  fallow,  namely,  Plot  4,  wheat 
alternate  with  cultivated  fallow;  Plot  5,  wheat  alternate  with  cloddy 


Fig.  6. — Crop  of  1920.  Wheat  continuously  on  the  left.  Wheat  after  fallow 
on  the  right.  Yields,  4  and  25  bushels  per  acre  respectively.  The  fallow  has 
a  very  marked  effect  on  wheat  growth  and  yield  during  dry  years. 

fallow,  and  Plot  7,  wheat  alternate  with  weedy  fallow.  The  yields 
on  these  plots  compared  with  their  adjacent  check  plots  bring  out  very 
strikingly  the  influence  of  the  summer  fallow  on  the  yield  of  wheat. 

TABLE   9 
The  Yield  of  Wheat  on  Alternate  Fallow,  Bushels  Per  Acre 


Plot:  Treatment 

Yield 
10-year 
period 

Yield 

lst-5  year 

period 

Yield 

2nd-5  year 

period 

3  wheat  continuously,  10  crops 

25.26 
40.07 
40  17 
24.95 
22.63 
40  12 
25.10 
15.02 

29.66 
37.66 
38.09 
32.49 
28.34 
37.87 
31.07 
6.7 

20.87 

43.7 

42.3 

6  wheat  continuously,  10  crop? 

17.4 

16.79 

Average  of  fallow  plots 

43.00 

Average  adjacent  checks 

19.13 

Difference  in  favor  of  fallow 

15.02 

BULL.  393]  CROP    SEQUENCES   AT   DAVIS  19 

In  this  comparison  it  is  interesting-  to  note  that  the  yields  on  the 
fallowed  plots  are  greater  during  the  second  five-year  period  than 
during  the  first,  a  condition  in  reverse  of  that  on  the  check  plots ;  and 
this  notwithstanding  the  fact  that  seasonal  rainfall  was  less  during 
the  second  half  of  the  period  than  during  the  first  half.  There  is  not 
sufficient  data  at  hand  to  enable  us  to  give  a  full  explanation  of  this. 
The  drying  of  the  soil  with  the  activities  and  changes  that  go  on 
during  that  process  may  partially  account  for  this  trend  of  results. 
Klein7  and  Kelly  and  McGeorge8  have  shown  that  both  drying  and 
heat  have  a  modifying  influence  on  the  productivity  of  soils.  The 
former  author  shows  that  under  humid  conditions  and  with  the  soils 
studied  the  drying  of  the  soil  increases  productivity  mainly  ' '  through 
changes  in  the  organic  content,  the  physical  nature  of  the  soil  and  the 
formation  of  nitrates.  Without  doubt  some  or  all  of  these  changes 
take  place  in  our  soils  during  the  process  of  summer  fallowing  and  the 
consequent  alternate  drying  and  wetting  tends  to  keep  them  produc- 
tive as  long  as  the  organic  matter  content  remains  within  suitable 
limits. 

It  should  be  noted  also  that  the  yield  of  the  check  plots  during 
the  second  period  are  much  reduced  because  of  the  very  low  yields 
obtained  from  the  harvest  of  1920  (see  table  7).  The  average  of  all 
check  plots  (18)  during  this  year  was  3.2  bushels  per  acre,  and  that 
of  these  two  adjacent  check  plots  (3  and  6)  was  4.25,  yet  the  average 
yield  of  wheat  on  plots  that  had  been  fallowed  the  previous  year  was 
26.1  bushels  per  acre.  The  indications  from  this  as  well  as  from  other 
evidence,  is  that  continuous  cropping,  even  for  a  short  period  results 
in  a  serious  depletion  of  yields  during  dry  years.  The  rainfall  for 
the  season  1919-20  was  only  8.94  inches — 8.29  inches  below  normal. 
The  factor  that  probably  prevented  complete  failure  that  year  was  the 
3.47  inches  of  rainfall  that  came  in  March. 

The  teaching  of  these  data  is  also  borne  out  by  the  yields  of 
wheat  on  plot  23,  where  a  fallow  preceded  the  wheat  crop  of  1920, 
which  yielded  25  bushels  per  acre ;  whereas  the  adjacent  check  plot  24, 
yielded  only  2  bushels  per  acre.  Likewise  on  plots  29  and  35  on 
which  barley  was  preceded  by  a  fallow  the  preceding  year,  the  barley 
crop  in  1920  was  43.6  bushels  and  41  bushels  per  acre  respectively; 
whereas  the  barley  crop  on  plot  25,  which  was  preceded  by  a  crop 
of  wheat,  was  that  year  only  11.6  bushels  per  acre. 

The  influence  of  the  summer  fallow  when  properly  made  is  further 
shown  by  the  yields  on  plots  19,  20  and  22,  wheat  2  years-fallow  and 
wheat  3  years-fallow,  with  their  adjacent  checks  18  and  21  as  follows : 


20 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE    10 

Influence  of  Frequency  of  Fallow  on  the  Succeeding  Crops  of  Wheat, 

Yields  in  Bushels  Per  Acre 


Plot 
18 

19  and  20 

21 

22 


Treatment 

Adjacent  check 

Fallow-wheat-wheat 

Adjacent  check 

Fallow-wheat-wheat-wheat 


Average  yield 
full  period 


22. 17  (10  crops) 
32.97(13  crops) 
23.70  (10  crops) 
29.22  (8  crops) 


Average  yield 
1st — 5  years 


28. 79  (5  crops) 
39.44  (6  crops) 
29.52  (5  crops) 
30.39  (4  crops) 


Average  yield 
2nd— 5  years 


15.56  (5  crops) 

27.4  (7  crops) 
17.88  (5  crops) 

28.05  (4  crops) 


On  plots  19  and  20  the  average  yield  of  the  six  crops  of  wheat 
following  the  fallow  was  38.03  bushels  per  acre,  while  the  average 
yield  of  the  seven  crops  preceding  the  fallow  was  28.52  bushels  per 
acre.  In  the  case  of  plot  22,  where  three  crops  of  wheat  occurred 
in  succession  a  similar  comparison  may  be  made.  The  average  of  the 
three  crops  following  the  fallow  was  41.2  bushels  per  acre.  The  yield 
of  the  three  crops  preceding  the  fallow  (third  crop  after)  was  29.29 
bushels  and  the  intervening  crop  averaged  for  the  three  years  17.2. 
The  fact  that  the  intervening  crop  was  less  than  that  of  the  third  year 
removed  from  the  fallow  is  not  easily  explained  but  may  have  been 
due  to  seasonal  conditions;  the  other  two  figures,  however,  are 
significant.  While  the  influence  of  a  fallow  every  third  or  every 
fourth  year  is  not  so  marked  as  the  fallow  every  alternate  year,  yet 
even  at  these  intervals  it  shows  a  favorable  influence  on  yields  over 
the  adjacent  check  plots.  Here  again  also  is  the  influence  of  a  dry 
year  shown,  for  plot  19  yielded  in  1920,  when  preceded  by  wheat, 
only  3.2  bushels  per  acre,  and  plot  22  yielded  under  the  same  circum- 
stances 4  bushels  per  acre. 

Barley  is  affected  in  a  similar  manner  by  a  well-made  fallow. 
Thus  plots  26,  28,  29  and  31  in  which  barley  follows  fallow  in  a 
rotation  of  barley-milo-wheat-fallow  gave  an  average  yield  from  9 
crops  of  60.03  bushels  per  acre ;  whereas  barley  preceding  fallow  on 
plots  23  and  25  gave  an  average  yield  from  6  crops  of  36.91  bushels 
per  acre.  On  most  of  these  plots  the  barley  crop  was  preceded  by  a 
wheat  crop.  On  plot  23  it  was  one  year  removed  from  the  previous 
fallow  and  on  25  it  was  2  years  removed.  When  these  figures  are 
compared  with  the  ten  crops  from  plot  8,  barley  continuously,  where 
the  average  yield  was  42.08  bushels  the  influence  of  a  fallow  on  barley 
yields  is  at  once  seen.  Also  in  another  rotation  (plots  32-37)  in 
which  barley  follows  fallow,  and  peas  turned  under  takes  the  place  of 
milo,  the  average  yield  from  nine  crops  of  barley  was  57.36  bushels 
per  acre. 


BULL.  393]  CROP    SEQUENCES   AT    DAVIS  21 

The  influence  of  fallow  on  the  yield  of  wheat  and  barley  is  striking, 
and  it  should  be  further  noted  that  on  all  plots  receiving"  fallow  treat- 
ment every  second,  third  or  fourth  year,  the  soil  is  in  better  physical 
condition  as  noted  by  the  ease  of  plowing  and  the  readiness  with 
which  the  soil  pulverizes.  There  is  no  measure  at  hand  for  this  effect, 
but  in  the  observation  of  all  who  have  come  in  contact  with  these 
plots,  the  fact  is  evident.  Moreover,  the  presence  of  a  summer  fallow 
in  the  rotation  helps  materially  in  keeping  the  land  clean  of  weeds. 
The  continuously  cropped  plots  are  noticeably  foul  with  weeds  and 
wild  oats. 

These  data  coupled  with  extensive  observations  suggest  three 
primary  benefits  to  be  derived  from  the  employment  of  a  properly 
prepared  summer  fallow  in  grain  production  under  the  climatic  and 
soil  conditions  under  study.  First,  the  cultivated  summer  fallow 
maintains  a  good  physical  condition  in  the  soil,  which  aids  in  the 
preparation  of  the  soil  and  the  control  of  weeds.  Second,  the  summer 
fallow  conserves  moisture  which  is  not  only  a  benefit  to  the  succeeding 
crop  but  favorably  influences  chemical  and  bacterial  action.  Third, 
fallow  land  is  well  prepared  for  planting  when  the  season  and  mois- 
ture conditions  are  most  suitable.  Timeliness  of  planting  is  an 
important  factor  in  grain  production  in  California;  for  if  the  plant- 
ing is  delayed  because  of  time  required  in  preparing  the  land  after 
the  fall  rains  begin,  the  crop  may  be  subjected  to  the  hazards  of 
unfavorable  weather  conditions  and  disease  infestation. 

Does  Summer  Fallowing  Pay. — The  question  arises,  however,  does 
the  increased  yield  of  wheat  or  barley  on  the  fallowed  land  bring 
sufficient  compensation  for  the  loss  of  the  land  to  crop  during  the 
period  of  the  fallow.  Upon  this  question  sufficient  data  for  a  precise 
answer  has  not  been  obtained;  but  it  is  believed  that  when  all  the 
benefits  of  fallow,  including  the  mainteance  of  producing  power  of 
the  land,  freedom  from  weeds,  improved  physical  condition  of  the 
soil  and  increased  yield  are  taken  into  consideration,  a  summer  fallow 
occurring  every  second  or  third  year  is  in  the  code  of  economic  prac- 
tices. If  $20  per  acre  may  be  considered  a  reasonable  cost  for  growing 
wheat  and  the  product  is  worth  one  dollar  per  bushel,  then  the  value 
of  the  wheat  above  the  cost  of  production  on  all  the  check  plots  for  the 
period  of  ten  years  has  been  $2.63  per  acre  per  year.  The  value  of 
the  wheat  on  the  three  plots  alternately  fallowed  has  been  $7.22  per 
acre  per  year  above  the  cost  of  production,  when  five  dollars  per  acre 
additional  is  allowed  for  making  the  summer  fallow.  It  is  realized 
that  this  is  not  a  complete  analysis  of  the  problem  but  it  serves  to 


22  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

indicate  that  if  the  producing  power  of  the  land  can  be  maintained 
at  a  high  level  a  fallow  every  other  year  may  not  be  unprofitable. 

Residual  Effect  of  Fallow. — How  often  should  a  summer  fallow 
occur  in  a  cropping  scheme  with  small  grains?  This  is  a  pertinent 
question  when  it  is  desired  to  secure  as  much  from  the  land  as  pos- 
sible consistent  with  the  maintenance  of  its  producing  power.  Sugges- 
tive data  may  be  secured  on  this  point  from  several  plots  in  the 
experiments,  which  are  set  forth  in  the  following  table: 

TABLE    11 

Eesidual  Effect  cf  Fallow  on  Wheat  Yields 

Yield  of  wheat  1st  year  after  fallow  43.39  bushels  per  acre,  average  11  crops 
Yield  of  wheat  2nd  year  after  fallow  30.18  bushels  per  acre,' average  9  crops 
Yield  of  wheat  3rd  year  after  fallow  26.06  bushels  per  acre,  average  10  crops 

The  figure  for  the  third  year  after  fallow  includes  the  yields  of 
wheat  after  milo  in  a  four-year  sequence  (plots  26-31)  :  fallow,  barley, 
milo,  wheat.  The  yields  of  wheat  in  the  second  year  after  fallow 
includes  the  yields  of  wheat  after  barley  in  a  three-year  sequence 
(plot  23),  fallow,  barley,  wheat.  The  indications  are  that  a  fallow 
every  fifth  year  will  not  be  of  any  particular  advantage,  so  far  as 
yields  are  concerned,  over  continuous  cropping.  It  is  interesting  to 
note  that  in  a  four-year  sequence  (plots  32-37),  where  wheat  three 
years  after  fallow  and  immediately  following  peas  turned  under, 
the  average  yield  of  wheat  for  six  crops  is  40.97  bushels  per  acre. 
If  these  six  crops  should  be  included  in  the  10  crops  averaged  above 
(3rd  year),  the  yield  is  brought  up  from  26.06  to  31.65,  thus  indicat- 
ing the  benefits  of  a  green  manure  crop. 

The  Influence  of  Manure. — It  has  often  been  observed  that  the 
use  of  barnyard  manure  in  grain  production  under  dry  farming 
conditions  does  not  produce  economic  returns ;  and  it  has  been  observed 
in  some  instances  to  actually  deplete  yields  (fig.  7).  Similar  results 
do  not  take  place  at  all  or  are  not  so  marked  under  irrigated  condi- 
tions. There  are  a  number  of  reasons  for  this,  some  of  which  are 
obscure.  But  it  is  quite  evident  that  under  conditions  of  a  limited 
moisture  supply,  too  much  carbon,  which  is  a  large  part  of  the  content 
of  barnyard  manure,  may  accumulate  in  the  soil,  and  thus  upset  the 
ratio  of  this  element  to  nitrogen  that  is  required  for  optimum  plant 
development.  Sievers  and  Holtzs  have  shown  that  straw  or  strawy 
material,  low  in  nitrogen  content,  turned  into  the  soil  under  conditions 
of  restricted  summer  rainfall  has  a  depressing  effect  on  yields  of 
grain.     This  effect  continues  until  the  ratio  of  carbon  to  nitrogen 


Bull.  393 


CROP    SEQUENCES    AT    DAVIS 


23 


is  brought  into  limits  suitable  to  the  crop's  demands  as  related  to 
kind  of  soil  and  moisture,  and  seasonal  conditions. 

In  this  experiment  one  plot  (13)  has  been  planted  with  wheat 
continuously  and  500  pounds  (5  tons  per  acre)  of  barnyard  manure 
added  each  year.  (1000  lbs.  were  applied  in  years  1919-22.)  The 
yields  of  this  plot  are  set  forth  in  the  following  table : 

TABLE    12 
Effect  of  Manure  on  the  Yield  of  Wheat — Bushels  Per  Acre 


Plot 

Treatment 

Average 
10  crops 

Average  yield 
1st — 5  years 

Average  yield 
2nd— 5  years 

13 

28.55 
24.98 

33.19 
31  05 

23  9 

12 

18.92 

Fig.  7. — Influence  of  green  manure  during  a  dry  year  (1920,  8.94  ins.). 
Plot  on  left  (38)  is  wheat  after  peas,  yield  30.5  bushels  per  acre.  Plot  on  right 
(39)  is  wheat  continuously  (check),  yield  3.4  bushels  per  acre.  In  the  right 
rear  may  be  seen  plot  41,  wheat  after  vetch,  yield  30.9  bushels  per  acre. 

These  results  indicate  a  slight  advantage  in  favor  of  the  manured 
plot  over  its  adjacent  check,  but  it  would  seem  that  the  increase  from 
the  use  of  manure  is  not  so  great  as  might  have  been  expected  if  there 
had  been  an  abundance  of  moisture  from  irrigation  or  rainfall. 

Influence  of  Seasonal  Rainfall. — Under  climatic  conditions  such 
as  prevail  at  Davis,  there  is  a  relation  between  the  use  of  manure 
and  available  moisture.  It  would  seem,  moreover,  that  the  available 
moisture  during  the  spring  or  growing  months  has  a  greater  influence 
on  yield  than  does  the  total  rainfall  or  seasonal  rainfall.  This  is 
illustrated  by  the  following  figures  where  the  yields  of  wheat  and 
rainfall  in  seasons  of  above  and  below  rainfall  are  compared : 


24 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE    13 

Eelation  of  Seasonal  and  Speing  Rainfall  (Feb. -April)  to  Yields  of  Wheat 
With  and  Without  Manure — Bushels  Per  Acre 


Five  years  above  normal  seasonal  rainfall 

Five  years  below  normal  seasonal  rainfall 

Seasonal 
rainfall 

Spring 
rainfall 

Average 

yield 

wheat  and 

manure 

plot  13 

Average 

yield 

plot  12 

(check) 

Average 
yield 

18 
checks 

Seasonal 
rainfall 

Spring 
rainfall 

Average 

yield 

wheat  and 

manure 

plot  13 

Average 

yield 

plot  12 

(check) 

Average 
yield 

18 
checks 

21.38 

6.41 

26.92 

23.73 

21.03 

13.30 

5.99 

32.76 

26.23 

24  10 

Six  years  above  normal  spring  rainfall 

Four  years  below  normal  spring  rainfall 

18.26 

8.10 

31.10 

29.82 

25.89 

15.90 

3.38 

24.71 

22.76 

21.21 

Thus  it  will  be  seen  that  the  average  yields  on  both  manured  and 
check  plots  were  higher  for  the  five  seasons  of  below  normal  seasonal 
rainfall  than  they  were  for  the  years  of  above  normal  seasonal  rainfall ; 
but  when  considered  on  the  basis  of  spring  rainfall  the  six  years  of 
above  normal  precipitation  gave  better  yields  than  the  four  seasons 
of  below  normal  rainfall  during  the  growing  months. 

These  figures  would  indicate  that  in  a  dry  climate,  such  as  that  at 
Davis,  there  is  a  possibility  of  getting  too  much  organic  matter  of  low 
nitrogen  content  and  possibly  other  materials  such  as  nitrates  as 
well.  Under  such  conditions  even  though  the  seasonal  rainfall  be 
normal  the  yields  are  materially  reduced  if  there  is  a  shortage  of 
precipitation  during  the  spring  months. 

A  similar  rotation  has  been  carried  on  at  Kearney  Vineyard,  near 
Fresno,  for  a  ten-year  period  covering  the  same  years  as  at  Davis. 
At  Kearney,  however,  the  yield  of  wheat  does  not  seem  to  be  influenced 
by  a  variation  of  seasonal  or  spring  rainfall  so  much  as  it  is  at  Davis. 
This  is  probably  due  to  the  fact  that  the  land  at  Kearney  receives  a 
supply  of  moisture  from  the  subsoil,  especially  during  the  summer 
months,  that  supplements  the  rainfall  during  periods  of  deficiency. 

TABLE    14 
Relation  of  Manure,  Fallow,  Continuous  Cropping  and  Rainfall  to  Yields 
of  Wheat  at  Kearney,  1913-23 

(Wheat  yields  bushels  per  acre.)     (Rainfall,  seasonal  normal  9.68  inches,  spring  normal  4.53  inches.) 


Wheat  in 
rotation 

with 
manure 

Wheat  in 

rotation 

no  manure 

Wheat 
after 
fallow 

Wheat 
continu- 
ously 

Seasonal 
rainfall 

Spring 

rai  nfall 

Feb.-April 

48.29 
40  14 
56.45 

35.92 
26.50 
51.35 

45.27 
34.81 
55.74 

25.02 
25  62 
24.43 

9.68 
9.84 
8.65 

4  53 

4  45 

4.10 

Bull.  393 


CROP    SEQUENCES   AT    DAVIS 


25 


The  rotation  employed  was  corn,  wheat,  beans ;  and  the  plots  were 
so  arranged  that  each  crop  was  grown  each  year,  though  not  on  the 
same  plot.  (In  1916,  however,  the  wheat  crop  was  cut  for  hay.) 
Manure  was  applied  to  one  series  at  the  rate  of  about  10  tons  per  acre. 
The  plots  were  of  *4  acre. 

During  the  eleven  years  period  (10  crops)  the  yield  of  wheat  on 
the  manure  rotation  was  12.37  bushels  per  acre  greater  than  on  the 
unmanured  rotation.  During  two  years,  however  (1919-1921)  the 
unmanured  plots  gave  higher  yields  than  the  manured  plots,  but  the 
difference  was  small.  During  these  two  years  both  the  seasonal  and 
spring  rainfall  was  below  normal ;  the  spring  rainfall  in  fact  was  but 
little  above  half  the  normal  for  these  years,  being  2.59  and  2.51  inches 
respectively. 

The  results  for  these  two  years,  with  the  influence  of  fallow  are 
set  forth  in  the  following  table : 


TABLE    15 

Yield  of  Wheat  on  Manured  and  Unmanured  Plots  and  After  Fallow  at 

Kearney  During  Dry  Years — Bushels  Per  Acre 


Year 

Wheat 
manured 

Wheat 
unmanured 

Wheat 
after  fallow 

Seasonal 
rainfall 

Spring 
rainfall 

1919  

41.93 
37.73 

43.20 
38.73 

51.43 
50.47 

6.90 
8.19 

2.59 

1921 

2.51 

Both  of  these  years  fall  in  the  second  period  and  their  yields  are 
lower  than  the  average  for  that  period.  However,  the  wheat  yields 
after  fallow  both  years  are  higher  than  on  the  manured  or  unmanured 
plots.  These  results  corroborate  the  results  generally  obtained ; 
namely,  that  a  well  prepared  summer  fallow  induces  or  maintains  a 
high  yield  of  wheat  during  dry  years.  While  there  are  considerable 
variations  in  yield  during  different  years  of  the  entire  period,  it  is 
of  interest  to  note  that  the  yields  under  all  treatments,  except  con- 
tinuous cropping,  have  been  greater  during  the  second  half  of  the 
ten-year  period  and  this  in  spite  of  the  fact  that  the  rainfall  for  both 
the  seasons  and  the  spring  has  been  slightly  lower  during  the  second 
than  the  first  half.  It  is  believed  that  the  rotation  employed  and  the 
better  physical  condition  of  the  soil  have  been  the  contributing  factors. 

The  comparison  of  manured  plots  and  unmanured  plots  is,  how- 
ever, of  striking  interest.  In  general  the  application  of  manure  has 
been  of  benefit,  but  evidently  the  increased  yield  has  not  been  sufficient 
to  pay  for  the  application.     The  difference  in  yield  of  wheat  between 


26 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


the  manured  and  unmanured  plots  was  much  greater  during  the  first 
half  of  the  period  (13.64  bushels  per  acre)  than  during  the  second 
half  (5.10  bushels). 

From  all  the  evidence  it  would  seem  that  with  the  heavy  application 
of  manure  each  season,  the  manure  is  being  applied  more  rapidly  than 
it  can  decay  under  the  light  rainfall  received.  The  accumulation  of 
undecayed  organic  matter  is  beginning  to  exert  a  retarding  influence 
on  the  crop.    The  application  of  manure  to  grain  land  under  a  deficient 


Fig.  8. — The  plot  on  the  right  (23)  is  continuously  cropped  to  wheat,  aver- 
age yield  for  10  crops  20.50  bushels  per  acre.  The  plot  on  the  left  is  wheat 
after  peas  turned  under  in  a  four-year  rotation.  Average  yield  for  ten  crops  42.34 
bushels  per  acre.     Crop  of  1919. 


moisture  supply  raises  some  interesting  problems.  The  amount  to 
apply  which  will  bring  the  best  results  depends  on  several  factors, 
including  stage  of  decomposition,  coarseness  and  presence  of  straw, 
moisture  supply  in  the  soil,  culture  practices,  nitrogen  content,  and 
the  like ;  but  in  all  events  manure  under  our  climatic  conditions  must 
be  applied  with  greater  understanding  and  care  than  under  humid 
conditions. 

It  should  be  noted  further  that  the  decline  in  yield  of  wheat  under 
continuous  cropping  (table  14)  is  not  so  marked  between  the  two 
periods  as  in  the  case  at  Davis,  but  that  continuous  cropping  results 
in  a  lower  yield  than  wheat  in  rotation  with  or  without  manure. 


Bull.  393 


CROP    SEQUENCES   AT    DAVIS 


27 


Effect  of  Green  Manure  on  Yield  of  Wheat. — In  this  experiment 
green  manure  crops  turned  under  have  had  a  pronounced  effect  at 
Davis  on  the  yield  of  wheat  (fig.  8).  This  is  brought  out  in  two 
rotations — one  of  wheat  alternated  with  peas  turned  under,  and  the 
other  of  wheat  alternated  with  vetch  turned  under.  Both  of  these 
rotations  consisted  of  two  plots  arranged  in  such  a  way  that  a  crop, 
both  of  wheat  and  legume  was  grown  each  year. 

The  results  are  set  forth  in  the  following  table : 

TABLE    16 
Effect  of  Peas  and  Vetch  on  Yield  of  Wheat 


Harvest  for  the  season 

Aver- 
age 

wheat 

Aver- 

Plot 

1914 

1915 

1916 

1817 

1918 

1919 

1920 

1921 

1922 

1923 

age 
legume 

38— Wheat-peas 

39— Check 

30.00 
35.00 

3.34 
24.00 
22.00 

3.31 

4.39 
15.00 
18.60 

6.10 
12.16 
15.92 

44.67 
21.67 

8.47 
54.33 
29.35 

6.05 

7.25 
30.60 
39.33 
10.25 
27.33 
36.00 

52.30 
25.50 
11.90 

51.60 
25.50 
10.00 

3.75 
16.90 
21.20 

1.75 
14.50 
21.20 

30.50 
3.40 
1.50 

30.90 
3.00 
1.25 

0  45 
14  20 
20.50 

0.25 
12.80 
22.80 

54.30 
28.00 

5.12 
47.70 
25.70 

5.25 

0.93 
15.30 
63.00 

4.43 
17.00 
68.80 

42.35 
20.56 
32.52 
41.70 
18.93 
32.94 

3.35 

40 — Peas-wheat 

41— Wheat- vetch 

42— Check 

6.06 
4  55 

43— Vetch-wheat 

5.17 

In  this  table  the  yields  of  wheat  are  expressed  in  bushels  per  acre 
and  the  yields  of  legumes  in  tons  of  green  matter  per  acre.  It  will  be 
noted  that  the  average  yield  of  wheat  after  green  manure  is  on  all 
four  plots  greater  than  the  yields  of  the  adjacent  check  plots.  Yet 
the  yields  of  wheat  on  plots  40  and  43  are  lower  than  on  plots  38  and 
41,  while  the  tonnage  of  green  matter  on  these  respective  pairs  of 
plots  is  the  reverse.  It  may  be  that  more  than  five  tons  of  legume 
turned  under  per  acre  is  an  excess  of  green  matter  under  these  con- 
ditions and  when  large  amounts  of  green  matter  are  turned  under 
yields  of  wheat  are  reduced. 

If  the  ten  crops  of  wheat  after  peas  and  the  ten  crops  of  wheat  after 
vetch  are  averaged  and  these  compared  with  adjacent  checks,  the 
influence  of  the  green  manure  is  more  strikingly  brought  out. 


TABLE   17 

Influence  of  Green  Manure  on  Yields  of  Wheat,  "Wheat-Bushels,  Peas  and 
Vetch — Tons  Green  Matter  Per  Acre 


Average  10  crops,  wheat  after  peas  37.44 
Average  10  crops,  peas  after  wheat  4.71 
Average  20  crops,  wheat  continu- 
ously (checks)  19.74 
Average  10  crops,  wheat  after  vetch 37. 34 
Average  10  crops,  vetch  after  wheat   4.86 


lst-5  crops  36.98  2nd-5  crops  37.9 

lst-5  crops  7.07  2nd-5  crops     2.35 

lst-5  crops  23.50  2nd-5  crops  15.08 

lst-5  crops  36.37  2nd-5  crops  38.28 

lst-5  crops  7.14  2nd-5  crops     2.58 


28  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

From  these  figures  it  will  be  noted  that  the  yields  of  wheat  after 
legumes  turned  under  is  considerably  augmented,  when  compared  with 
wheat  grown  continuously.  Also  that  the  yields  of  wheat  after  legumes 
was  slightly  greater  the  second  half  of  the  period  than  the  first  half, 
although  the  rainfall  during  that  period  was  below  normal.  The 
amount  of  green  matter  turned  under  per  acre  was,  however,  consider- 
ably less  during  the  second  half  of  the  period  than  the  first. 

The  influence  of  peas  turned  under  is  also  brought  out  in  another 
rotation  consisting  of  four  plots  (32-37)  with  checks  and  arranged  so 
that  a  crop  of  barley,  peas,  wheat,  followed  by  fallow  was  harvested 
each  year. 

The  results  are  tabulated  as  follows : 

TABLE    18 

Influence  of  Peas  cn  Wheat  in  a  Four-Year  Botation.      Wheat  Yield  in 
Bushels.     Peas  in  Tons  Green  Matter  Per  Acre 

Average  yield  of  10  crops  of  wheat 

after  peas  42.34    lst-5  crops    41.49    2nd-5  crops  43.20 

Average  yield  of  10  crops  of  peas         5.06     lst-5  crops      6.09    2nd-5  crops     4.03 
Average  yield  of  20  crops  of  wheat 

adjacent  check  plots  33.36  19.90     lst-5  crops    24.88    2nd-5  crops   14.93 

These  figures  show  a  slightly  increasing  yield  of  wheat  following 
peas  turned  under  during  the  second  half  of  the  ten-year  period, 
though  the  average  yield  of  peas  turned  under  is  very  materially 
decreased.  The  yield  of  wheat  in  this  rotation  is  somewhat  higher 
than  in  the  rotation  of  alternate  wheat  and  peas.  This  would  indicate 
that  a  legume  turned  under  once  in  four  years  with  also  a  fallow 
period  is  better  than  a  legume  turned  under  every  alternate  year.  It 
is  recognized  that  this  may  not  be  an  economic  practice  and  such 
results  might  not  be  obtained  if  the  legume  were  harvested  for  forage 
or  seed. 

The  yields  of  barley  in  this  rotation  are  also  maintained  at  a 
higher  level  than  those  on  the  continuously  cropped  plots,  but  it  is 
believed  that  in  this  case,  barley  following  fallow,  the  crop  is  more 
influenced  by  the  previous  fallow  than  it  is  of  the  crop  of  peas  turned 
under  two  years  previously. 

The  data  of  this  rotation  as  well  as  general  observation  would 
indicate  that  a  four-year  sequence  of  crops  with  one  year  legumes  and 
one-year  fallow  will  maintain  the  crop  producing  power  of  the  soil 
at  a  high  level  under  the  conditions  existing  at  Davis.  How  profitable 
this  practice  would  be,  would  depend  on  many  conditions,  related  to 
stand,  value  and  use  of  the  legume  crop. 


BULL.  393]     .  CROP    SEQUENCES    AT    DAVIS  29 

Wheat  and  Legume  Combined. — One  plot  in  the  series  (53)  has 
grown  wheat  continuously  with  a  light  seeding  of  burr  clover  or 
Hubam  in  the  spring.  The  stand  of  the  legume  has  varied  widely 
during  the  ten-year  period,  and  continuous  record  of  the  amount  of 
dry  matter  in  legume  restored  to  the  soil  has  not  been  kept.  However, 
the  results  do  not  show  an  advantage  in  planting  these  legumes  with 
wheat.  The  average  yield  of  the  wheat  crop  for  the  ten-year  period 
was  20.56  bushels  per  acre  with  an  average  of  23.49  bushels  during 
the  first  half  of  the  period  and  17.6  bushels  during  the  second  half. 
This  plot  showed  a  very  low  yield  during  the  dry  year  of  1920,  as  did 
the  check  plots.  The  records  for  the  adjacent  check  were:  average  for 
the  ten-year  period  20.34 ;  first  half  of  period,  25.98  and  for  the  second 
half,  14.70  bushels  per  acre.  About  the  only  advantage  to  be  noted  in 
this  combination,  is  that  the  yield  during  the  second  half  of  the  period 
has  not  declined  so  rapidly  as  on  the  adjacent  check,  or  18  checks 
combined. 

Comparison  of  Biennial  Cropping  With  and  Without  Green 
Manure. — By  tabulating  the  yields  from  plots  in  various  parts  of  the 
experiment  it  is  possible  to  compare  the  influence  of  green  manures 
and  fallow  in  increasing  yields.  For  this  comparison,  plot  38,  wheat- 
peas,  and  plot  41,  wheat-vetch,  have  been  chosen  as  representative  of 
the  green  manure  plots,  and  plots  4  and  5  as  representative  of  the 
fallow  plots,  because  all  of  these  plots  were  in  wheat  the  same  years 
throughout  the  period.  In  the  following  table  the  average  yields  of 
wheat  of  these  plots  are  given  for  the  ten-year  period  only,  comprising 
five  crops  in  each  case,  and  the  same  years  are  chosen  from  the  checks 
as  were  in  wheat  on  the  other  plots. 

TABLE    19 

Comparison  of  Green  Manure  and  Fallow  in  Their  Influence  on  the  Yield 
of  Wheat.     Average  Yield  in  Bushels  Per  Acre 

Wheat  on  plot  38 — wheat-peas 42.35 

Wheat  on  plot  39 — wheat  continuously 22.71 

Wheat  on  plot  41 — wheat-vetch 41.70 

Wheat  on  plot  42 — wheat  continuously 19.  31 

Wheat  on  plot    3 — wheat  continuously 24.97 

Wheat  on  plot    4 — wheat-fallow 40.07 

Wheat  on  plot    5 — wheat-fallow 40. 17 

Wheat  on  plot    6 — wheat  continuously 24.62 

Average  yields  on  green  manure  plots  (38,  41) 42.02 

Average  yields  on  their  adjacent  checks  (39,  42) 21.01 

Average  yields  on  fallow  plots  (4,  5) 40. 12 

Average  yields  on  their  adjacent  checks  (3,  6) 24.80 

Average  difference  in  favor  of  green  manure  over  adjacent  checks 21.01 

Average  difference  in  favor  of  fallow  over  adjacent  checks 15.32 


30  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

It  will  be  noted  from  these  data  that  for  a  ten-year  period  there 
is  a  difference  in  yield  of  wheat  with  fallow  and  with  green  manure 
in  a  biennial  cropping  system,  and  the  indications  are  that  as  time 
goes  on  the  difference  will  increase  in  favor  of  the  green  manure  plots, 
because  even  now  the  soil  on  the  green  manure  plots  appears  more 
friable  and  in  better  physical  condition,  than  that  of  the  fallow  plots 
or  the  adjacent  checks.  The  measure  of  economy  in  these  trials  is  not 
shown  by  the  figures,  and  except  under  special  conditions  the  practice 
of  green  manuring  as  here  indicated  may  not  be  economical. 

The  Effect  of  Milo  in  Rotation  With  Small  Grain. — A  four  course 
rotation  with  barley,  milo,  wheat  and  fallow  has  been  carried  out  on 
plots  26-31.  This  sequence  is  interesting  both  from  the  standpoint  of 
the  feasibility  of  producing  a  profitable  crop  of  grain- sorghum  without 
irrigation;  and  also  from  the  standpoint  of  the  effect  of  a  cultivated 
crop  compared  with  a  summer  fallow.  The  yield  data  from  the  ten- 
year  period  are  given  in  the  following  table : 

TABLE  20 
Yield  of  Milo  in  Botation  and  Continuously,  Bushels  Per  Acre 

Milo  in  4-year  rotation,  average  10- 
year  period  56.11     lst-5  years    68.16    2nd-5  years  46.5 

Milo  continuously,  average  10-year 

period  50.06     lst-5  years    73.34    2nd-5  years  31.44 

Thus,  it  is  indicated  that  milo  holds  up  better  in  a  rotation  which 
includes  a  fallow  than  it  does  under  continuous  cropping.  It  should 
be  noted,  however,  that  under  both  summaries,  the  crop  of  '18  is 
not  included  because  the  early  fall  rains  and  excessive  bird  damage 
left  no  crop  to  harvest  that  year. 

Summer  Fallow  and  Moisture. — The  summer  fallow  seems  to  have 
at  least  three  direct  benefits  on  the  crop  of  grain  which  is  to  follow. 
If  properly  made  and  timed  it  conserves  moisture,  it  causes  an  elabor- 
ation of  nitrates,  and  the  land  is  ready  to  plant  when  the  proper 
seasonal  and  moisture  conditions  arrive. 

Moisture  determinations  were  made  on  all  plots  of  this  experiment 
during  the  years  1914—17  inclusive.  Samples  were  taken  to  a  depth  of 
six  feet  on  all  plots  and  deeper  on  some  plots,  and  the  moisture  cal- 
culated as  percentage  of  dry  soil.  In  the  following  table  the  moisture 
content  on  all  plots  fallowed  during  the  year  indicated  is  shown,  and 
for  comparison  the  moisture  content  on  the  check  plots  adjacent  to 
the  fallow  plots  is  also  shown. 


Bull.  393 


CROP    SEQUENCES    AT    DAVIS 


31 


TABLE    21 

Moisture  Percentages   to  a   Depth   of   Six   Feet   on    Fallowed   Plots   and 
Cropped  Plots  (Checks)  at  the  Close  of  the  Summer  Season  for  Four  Years 


Sampled  Oct.  30.  1914 

Sampled  Oct.  29,  1915 

Sampled  Nov.  14,  1916 

Sampled  Oct.  30,  1917 

Plot 

Fallow- 

Plot 

Ad- 
jacent 
checks 

Plot 

Fallow 

Moist- 
ure 

Plot 

Ad- 
jacent 
checks 

Plot 

Fallow- 

Plot 

Ad- 
jacent 
checks 

Plot 

Fallow 

Plot 

Ad- 
jacent 
checks 

Moist- 
ure 

Moist- 
ure 

Moist- 
ure 

Moist- 
ure 

Moist- 
ure 

Moist- 
ure 

Moist- 
ure 

20 
31 
37 

21.40 
17.98 
18.53 

21 
30 

36 

14.59 
14.57 
15.03 

4 
19 
29 
35 

13.35 

19.92 
18.64 
18.67 

3 
18 

30 
36 

8.66 
14.51 
12.26 
10.16 

17 

23 
28 
34 

19.74 
19.74 
18.07 
18.83 

18 
24 
27 
33 

18.17 
16.80 
15.56 
14.57 

4 

20 
22 
25 
26 
32 

14.98 
19.57 
19.01 
20.22 
19.17 
18.19 

3 
21 

24 
27 
33 

15.61 
15.82 

12.86 
14.94 
17  40 

Av. 
Dif. 

19.30 
4  57 

14.73 

17  61 
5.24 

11.40 

19.09 
2  82 

16.27 

18.52 
3.20 

15  32 

The  data  at  hand  is  inadequate  to  explain  the  low  differences  for 
the  year  1916  and  1917;  but  notwithstanding  this  fact  the  amount 
of  moisture  conserved  is  significant  in  its  bearing  on  the  crop  the 
following  year,  for  it  will  be  noted  that  the  average  yield  of  wheat 
in  1917  on  plots  17  and  23,  which  had  been  fallowed  the  summer  of 
1916,  was  55.41  bushels  per  acre,  while  the  average  yield  of  check 
plots  18  and  24  for  the  same  season  was  43.74  bushels,  or  a  gain  of 
more  than  eleven  bushels  per  acre  in  favor  of  fallow.  Similarly  the 
average  yield  of  wheat  in  1918  on  plots  4,  20  and  22  after  fallow,  was 
46.1  bushels  per  acre,  whereas  the  average  yield  on  the  adjacent  check 
plots  3  and  21  was  31.4  bushels. 

Thus  it  seems  quite  evident  both  from  these  data  as  well  as  from 
general  observation  that  a  properly  made  summer  fallow  conserves 
moisture  to  the  advantage  of  the  following  crop.  Whether  this  advant- 
age is  due  directly  to  the  moisture  conserved  or  rather  to  the  more 
favorable  distribution  of  moisture  in  the  soil  column  or  to  the  influence 
of  this  moisture  on  the  chemical  and  bacterial  activities  in  the  soil, 
it  is  not  possible  to  say.  It  is  well  known  that  both  bacterial  and 
chemical  forces  are  more  active  in  a  fallow  soil  during  the  summer 
months  than  under  stubble  where  conditions  are  otherwise  similar,  and 
that  as  a  result  of  these,  the  fallow  soil  is  improved  in  its  crop 
producing  power.  Sievers  and  Holtz9  in  studies  of  silt  loam  soils  and 
their  yields  in  Eastern  Washington  came  to  the  conclusion  that  mois- 
ture conservation  under  summer  fallow  practices  serves  its  primary 
purpose  in  the  elaboration  of  nitrates.    Specifically  they  state  (p.  19)  : 


32  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

"Available  soil  nitrogen  and  not  total  soil  moisture  is  the  limiting 
factor  in  crop  production  in  this  region  and  that  the  real  basis  for 
summer  fallowing  is  to  make  the  maximum  amount  of  nitrogen  ready 
for  plant  use. ' ' 

The  data  at  hand  and  our  observations  indicate  that  similar  con- 
clusions can  be  drawn  for  the  conditions  of  the  present  study,  and 
that  the  uniform  distribution  of  moisture  under  summer  fallow  within 
optimum  limits  for  bacterial  and  chemical  action,  is  of  greater  signifi- 
cance than  the  total  amount  conserved.  Emphasis  must  be  placed, 
however,  on  the  primary  importance  of  moisture,  for  it  is  upon  this 
that  nitrate  increase  depends.  As  shown  in  table  22  below,  the 
moisture  under  the  fallow  is  not  only  higher  than  under  the  checks, 
but  also  more  uniformly  distributed ;  and  likewise  the  yields  of  wheat 
are  higher  on  plots  fallowed  than  on  the  checks. 

Trend  of  Moisture  Under  Continuous  Cropping  and  Fallow. — For 
the  four  years,  1914-17  inclusive,  moisture  determinations  on  all  plots 
were  made  to  a  depth  of  six  feet.  In  general  the  percentage  of 
moisture  for  each  foot  declines  as  the  season  advances  until  late  in  the 
summer  or  early  in  the  fall  when  there  is  a  recovery,  even  before  the 
rains  set  in.  During  the  summer  months  currents  of  capillary  moisture 
are  set  upward  while  the  evaporation  from  the  soil  surfaces  or  trans- 
piration from  leaf  surface  is  at  the  maximum.  When  these  losses  are 
retarded  toward  the  latter  part  of  the  summer,  the  currents  of  capillary 
moisture  from  the  lower  depths  continue,  and  thus  the  moisture  con- 
tent of  the  entire  soil  column  is  increased.  This  tendency  is  illustrated 
by  the  figures  in  table  22  (below)  in  which  the  percentage  of  moisture 
in  each  of  the  upper  six  feet  of  soil  are  averaged.  It  will  also  be 
noted  that  the  fifth  foot  generally  contains  less  moisture  than  either 
the  fourth  or  the  sixth. 

The  influence  of  the  summer  fallow  on  the  conservation  of  moisture 
may  be  further  indicated  by  the  data  contained  in  this  table.  The  data 
is  for  the  season  1915-16.  During  this  season  samples  were  taken  to 
a  depth  of  6  feet  on  five  different  dates,  at  two-  and  three-month 
intervals.  During  this  season  the  rainfall  was  a  little  above  normal 
(3.65  inches)  but  only  .21  of  an  inch  fell  after  the  first  samples  were 
taken  April  1st.  For  this  comparison  the  four  fallow  plots  (17,  23, 
28,  34)  of  that  season  were  taken  with  their  adjacent  check  plots  (18, 
24,  27,  33). 

These  figures  indicate  that  owing  to  a  relatively  moist  soil  at  depths 
below  6  feet  the  entire  six-foot  column  has  been  pretty  well  supplied 
with  moisture  under  both  crop  and  fallow.  The  first  foot  on  the  check 
plots  shows  a  greater  tendency  to  dry,  but  in  every  instance  the  fallow 


Bull.  393" 


CROP    SEQUENCES   AT    DAVIS 


33 


plots  show  a  greater  percentage  of  moisture  than  do  the  check  plots 
for  each  comparable  foot,  and  in  general  this  same  relationship  holds 
true  for  the  seventh  foot  and  the  eighth. 


TABLE    22 

Trend  of  Moisture  Under  Continuous  Cropping  and  Fallow.  Season  1915- 
1916,  for  Each  Foot  up  to  Six  Ft.  Depth.  Samples  Taken  on  Dates 
Indicated,  Moisture  in  Percentage  of  Dry  Soil. 


April  1 

May  25 

July  7 

August  29 

November  4 

Fallow 

Checks 

Fallow 

Checks 

Fallow 

Checks 

Fallow 

Checks 

Fallow 

Checks 

1  foot 

21.61 
23.50 

27.77 

20.24 
22.69 
28.34 

16.86 
21.51 
26.85 

13.30 
14.31 
19.73 

17.27 
21.81 

25.77 

9.91 
13.54 
20.75 

12.99 
19.59 
23.39 

7.85 
12.59 
18.92 

13.86 
19.71 
23.72 

£.20 

2  feet 

16.36 

3  feet 

23.23 

Average  3  feet.... 

24.29 

23.76 

21.74 

15.78 

21.61 

14.13 

IB.  66 

13.12 

19.  C9 

16.26 

4  feet 

30.18 
30.08 

32.76 

30.04 
29.35 
33.27 

28.75 
24.77 
30.58 

26.20 
27.48 
26.28 

26.95 
26.49 
30.27 

25.71 
22.26 
29.32 

24.43 
22.42 
30.27 

22.89 
21.95 
28.31 

24.75 
23.95 
27.85 

25.19 

5  feet 

22.82 

6  feet 

26.66 

Average  3  feet... 

31  01 

30.88 

28.03 

26.65 

27.90 

25.76 

25.11 

24-38 

25.52 

%!r.b9 

Average  6  feet  .. 

27.65 

27.32 

24.88 

21.21 

24.76 

20.25 

22.18 

18.75 

22.30 

20.57 

Some  Lessons  From  a  Dry  Year. — The  season  of  1919-20  was 
exceptionally  dry,  not  only  in  respect  to  total  rainfall,  which  was 
only  8.94  inches  (8.29  inches  below  normal),  but  also  in  respect  to 
spring  rainfall  (Feb.  Mar.  and  April),  which  was  only  5.10  inches. 

The  effect  of  this  shortage  of  moisture  became  noticeable  by  the 
end  of  March,  at  which  time  the  grain  on  fallow,  and  that  after  green 
manure,  stood  out  thrifty  and  vigorous  compared  with  that  on  the 
continuously  cropped  plots  (checks)  and  on  the  manure  plot  (13). 
By  the  middle  of  April,  failure  could  be  seen  for  all  checks  and  con- 
tinuously cropped  plots,  while  those  on  fallow  and  green  manure  were 
still  promising. 

Some  yields  during  this  year  are  interesting  and  are  set  forth 
in  the  following  table. 

Evidence  of  the  advantage  of  fallow  and  green  manures  during  this 
dry  year  is  striking  not  only  in  relative  yields  but  also  in  weights  per 
bushel  of  the  grain  harvested.  In  general  where  yields  are  lowest 
the  grain  is  lighter.  These  data  corroborate  to  a  certain  extent  the 
fact  that  the  duty  of  water  or  moisture  is  greater  in  soils  of  high 
fertility,  or  crop  producing  power.  The  relative  drought  resistance  of 
the  several  crops  above  listed  is  also  indicated. 


34  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

TABLE    23 

Some  Yields  During  the  Dry  Year,  1920 

Bushels  Pounds  per 

per  acre  bushel 

Average  yield  of  wheat  on  all  checks  (18) 3.2  49.83 

Barley  continuously,  plot  8 16.4  45. 

Rye  continuously,  plot  10 6.6  54 

Oats  continuously,  plot  11 4.8  31 

Milo  continuously,  plot  14 2.3 

Wheat  with  legume  continuously,  plot  53 2. 1  44 

Wheat  with  manure  continuously,  plot  13 2.2  38 

Wheat  after  fallow,  average  5  plots 28.34  56.2 

Barley  after  fallow,  average  2  plots 42.3  40 

Wheat  after  peas  turned  under,  plot  38 30.  5  56.  5 

Wheat  after  vetch,  average  2  plots 30.5  58 


SUMMARY 

With  an  annual  rainfall  of  twenty  inches  or  less,  change  of  crops 
and  the  inclusion  of  a  fallow  period  are  important  factors  in  the 
maintainance  of  high  yields. 

In  1913  a  series  of  plots  was  laid  out  at  Davis,  California,  These 
plots  were  designed  to  ascertain  the  influences  of  certain  crop 
sequences,  summer  fallow,  green  manures,  legumes  and  continuous 
cropping  on  the  crop  producing  power  of  Yolo  loam  and  Yolo  silt 
loam,  the  two  soil  types  over  which  the  plots  extend. 

The  seasonal  and  spring  rainfall  exert  an  important  influence  on 
the  yield  of  cereal  crops;  and  of  the  two  influences,  spring  rainfall  is 
the  more  important. 

On  the  whole,  the  Yolo  loam  seems  to  be  more  productive  of  wheat 
under  the  conditions  than  the  Yolo  silt  loam. 

In  1916,  when  the  annual  rainfall  was  21.63  inches,  the  moisture 
under  the  eighteen  check  plots  (wheat  continuously)  to  a  depth  of 
six  feet,  was  reduced  from  an  average  of  26.88  per  cent  on  April  1 
to  an  average  of  17.55  per  cent  on  September  1.  On  November  1  the 
moisture  content  had  risen  to  an  average  of  19.67  though  no  rainfall 
had  been  received  between  the  two  last  samplings.  This  increase 
previous  to  the  last  sampling  was  noted  for  each  succeeding  foot  except 
the  fourth. 

Continuous  cropping  decreases  the  yield  of  cereals ;  and  the 
decrease  is  augmented  with  the  passage  of  time.  This  conclusion  is 
borne  out  by  experiments  under  many  other  climatic  and  soil  con- 
ditions. The  depressing  effect  of  continuous  cropping  is  especially 
marked  during  dry  years. 


Bull.  393]  CROP    SEQUENCES   AT   DAVIS  35 

Cultivated  summer  fallow  tends  to  maintain  high  yields.  It 
should  not  be  assumed,  however,  that  this  maintainance  will  be  con- 
tinuous over  a  long  period  of  time.  The  principal  influence  seems  to 
be  due  to  a  more  equal  distribution  of  moisture  rather  than  to  the 
total  amount  conserved. 

The  economy  of  summer  fallow  depends  on  its  frequency,  the 
presence  of  legumes  or  cultivated  crops  in  the  rotation  and  the  amount 
and  distribution  of  rainfall.  In  general,  however,  yields  decline  as 
the  period  between  fallow  years  increases. 

Manure,  if  too  coarse  and  if  applied  too  abundantly,  may  be  detri- 
mental, especially  during  dry  years.  Its  effect  is  related  among  other 
things,  to  its  composition  and  to  the  moisture  supply. 

Green  manures  of  peas  and  vetch  maintain  the  yields  of  wheat  at 
a  high  level,  but  the  results  would  indicate  that  during  dry  years  too 
much  can  be  added  for  the  best  results.  The  optimum  amount  may 
be  around  five  tons  per  acre. 


LITERATURE  CITED 

i  Holmes,  L.  C,  and  Nelson,  J.  W. 

1913.  Reconnaissance  soil  survey  of  the  Sacramento  Valley  area,  California. 
U.  S.  Dept.  Agr.  Advance  Sheets  Field  Operations  Bur.  Soils. 
91-93. 

2  Brezeale,  J.  F. 

1924.  The  injurious  after  effects  of  sorghum.     Jour.  Am.  Soc.  Agron.     16: 

689-700. 

3  Hawkins,  R.  S. 

1925.  The  deleterious  effect  of  sorghum  on  the  soil  and  on  the  succeeding 

crops.     Jour.  Am.  Soc.  Agron.  17:  91. 

*  Schreiner,  Oswald,  and  Reed,  H.  S. 

1909.     The  isolation  of  harmful  organic  substances  from  soils.     U.  S.  Dept. 
Agr.  Bur.  Soils  Bull.  53:   51-53. 

5  Madson,  B.  A. 

1916.      A    comparison    of    annual    cropping,    biennial    cropping    and    green 
manures  on  the  yield  of  wheat.     California  Agr.  Expt.  Sta.  Bull. 
270:  1-14. 
e  Klein,  M.  A. 

1915.     Studies  in  the  drying  of  soils.     Jour.  Am.  Soc.  Agron.     7:   72-75. 
7  Kelly,  W.  P.,  and  Mc.  George,  W. 

1913.     The  effect  of  heat  on  Hawaiian  soils.     Hawaii  Agr.  Expt.  Sta.  Bull. 
30:  5-7. 

s  Sievers,  F.  J.,  and  Holtz,  H.  F. 

1922.     The  silt  loam  soils  of  Eastern  Washington  and  their  management. 
Washington  Agr.  Expt.  Sta.  Bull.  166:  56:59. 
s  Sievers,  F.  J.,  and  Holtz,  H.  F. 

1922.  The   silt   loam  soils   of  eastern   Washington   and  their   management. 
Washington  Agr.  Expt.  Sta.  Bull.  No.  166. 


36  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


ACKNOWLEDGMENTS 

The  experiments  upon  which  the  data  and  discussions  of  this  bul- 
letin are  based,  and  during  the  period  under  discussion,  have  been 
under  the  immediate  charge  of  Professors  Madson  and  Hendry  and 
Mr.  Conrad ;  and  it  is  to  them  that  credit  is  due  for  carrying  out  the 
details  of  the  experiments.  In  working  out  the  plan  and  method  of 
procedure  valuable  assistance  was  rendered  by  Professors  Madson 
and  Hendry.     The  photographs  were  taken  by  Professor  Hendry. 


15m-10,'25 


